1. Field of the Invention
This invention relates generally to a semiconductor laser device which can continuously emit a laser light of a short wavelength band at a room temperature and, more particularly, is directed to an AlGaInP-system semiconductor laser.
2. Description of the Prior Art
Recently, a demand for a semiconductor laser device which can continuously emit a laser light of short wavelength band at the room temperature has occurred.
In, for example, an optical disc such as a so-called compact disc (CD), a video disc and the like, if a laser light of short wavelength is employed as the light source thereof, high density recording becomes possible and also it becomes possible to use a lens system having a small numerical aperture (NA) so that the optical system can be produced at a low cost. Further, since the light from a short wavelength light source is visible light it becomes easy and safe to handle the semiconductor laser. Furthermore, in a laser printer and the like, the photosensitive material has a has high sensitivity in the range of the visible spectrum so that it becomes advantageous to use the short wavelength semiconductor laser as a light source for exposure. For this reason, it is expected that the short wavelength semiconductor laser which can continuously emit a light at room temperature will be produced.
In order to construct the short wavelength semiconductor laser, it is necessary to increase the band gap width of an active layer thereof. In this case, when the material of this active layer is selected, there are various kinds of restrictions, for example, it is necessary to consider a cladding layer for confining a carrier and light into the selected active layer and it is also necessary to consider, from a crystal property standpoint, a lattice constant of a crystal substrate on which the layers are epitaxially grown.
With respect to the short wavelength semiconductor laser of this kind, an AlGaInP/GaAs-system semiconductor laser in which a semiconductor layer of AlGaInP-system is epitaxially grown on an GaAs substrate to thereby construct a semiconductor laser attracts attention as a semiconductor laser with a so-called wavelength band of 600 nm which lies in a range from 580 nm to 680 nm.
This AlGaInP/GaAs semiconductor laser is constructed such that as shown in FIG. 1, on one major surface of a GaAs substrate 1, there are epitaxially grown a first cladding layer 2, an active layer 3, a second cladding layer 4 and a capping layer 5 in this sequentially order. Reference numeral 6 designates an electrode which is deposited in ohomic contact manner on other major surface of the GaAs substrate 1 and reference numeral 7 designates an electrode which is deposited on the capping layer 5 in an ohmic contact fashion. The electrode 7 itself is arranged to have a function as a heat sink. Alternatively, the electrode 7 is soldered through a solder layer 8 to a heat sink, that is, a radiator 9.
The layers 2 to 4 are made of material having the composition shown by the following general formula. EQU (A1.sub.x Ga.sub.1-x).sub.y In.sub.1-y P (1)
The active layer 3 is made of Ga.sub.0.52 In.sub.0.48 P having a thickness of, for example, 0.1 .mu.m to 0.2 .mu.m. The first and second cladding layers 2 and 4 are each made of an AlGaInP-system semiconductor layer having a band gap width (forbidden band width) larger than that of the active layer 3 by about 0.3 eV. Specifically, they are each made of, for example, Al.sub.0.26 Ga.sub.0.26 In.sub.0.48 P of which the Al content (value of x in formula (1)) is larger than 0.3, more preferably larger than 0.5. Each thickness of these layers 2 and 4 is selected to be more than 0.8 .mu.m which is thick enough to confine the carrier and the light within the active layer 3, for example, 1 .mu.m. Further, the capping layer 5 is made of, for example, a GaAs layer.
The AlGaInP/GaAs-system semiconductor laser with the above construction emits a laser light of a short wavelength of, 653 nm. In this case, this semiconductor laser can continuously emit a laser light at a temperature of 228.degree. K. but can not do it at room temperture.
The reason that this semiconductor laser can not continuously emit the laser light at room temperature is because the heat generated in the active layer 3 during operation is not conducted effectively to the heat sink 9.
For example, in a semiconductor laser, with respect to the InGaAsP/GaAs-system, a semiconductor laser formed as a mesa-type structure and thereby capable of continuously emitting a laser light at room temperature is disclosed, for example, in "Nikkei Electronics, 1985, May 20, PP. 151 to 153", "Nikkei Micro Device, 1985, summer edition, PP. 21 to 23" and "Electronics Letters, 1985, January 17, Vol. 21, No. 2, PP. 54 to 56". According to this mesa-type semiconductor laser, grooves are formed, which enter a cladding layer on which a metal electrode, which becomes a heat sink, is deposited. On the upper surface of the mesa-type semiconductor laser opposite to the GaAs substrate, the metal electrode is deposited so as to include the inside of these grooves, whereby the metal electrode having the heat sink effect approaches the active layer as near as possible. Thus, the heat generated from the active layer can be radiated effectively to thereby enable the semiconductor laser to continuously emit laser light at room temperature.
However, the fact that the electrode exists very near the active layer as described above causes problem of a leakage current. Further, from a technical standpoint, it is difficult to form such grooves so so that they are spaced apart from the active layer by a proper distance and with a proper depth to sufficiently approach the active layer without the occurrence of leakage current. There is a problem that the semiconductor laser having a uniform characteristic cannot mass-produced.